It is known from, for example, U.S. Pat. No. 6,829,888 that an electric machine can be used to assist with heating the exhaust gas flow from an engine to assist with catalyst light-off.
The use of high pressure exhaust gas recirculation (HPEGR) or low pressure exhaust gas recirculation (LPEGR) at the earliest possible moment during a vehicle drive cycle helps to reduce NOx emissions. Early introduction of EGR after engine starting has been used to reduce NOx emissions to satisfy emission requirements. The engine operating range in terms of engine speed and engine load and the temperature range over which EGR flow is used to reduce NOx have both increased to meet more recent emission requirements.
One problem associated with the early use of EGR is that it can result in combustion instability if the temperature of the gas inducted into the engine is low such as following a cold start.
A second problem associated with the early use of LP EGR in the case of an engine having forced induction using a compressor is that it can result in the formation of condensation upstream of the compressor. The formation of such condensation can seriously damage the fast rotating blades of the compressor.